WASHINGTON  Earth's protective ozone layer, which was notably thinning in 1980, may be fully recovered by mid-century, climate scientists said Wednesday.

Ozone in the stratosphere, outside the polar regions, stopped thinning in 1997, the scientists found after analyzing 25 years worth of observations.

The ozone layer shields the planet from the sun's harmful ultraviolet radiation, but human-made chemicals -- notably the chlorofluorocarbons found in some refrigerants and aerosol propellants -- depleted this stratospheric ozone, causing the protective layer to get thinner.

The scientists said the ozone layer's comeback is due in large part to compliance with an 1987 international agreement called the Montreal Protocol, which aimed to limit emissions of ozone-depleting chemicals.

"These results confirm the Montreal Protocol and its amendments have succeeded in stopping the loss of ozone in the stratosphere," said Eun-Su Yang of the Georgia Institute of Technology, who led a team that analyzed the data.

"At the current recovery rate ... the global ozone layer could be restored to 1980 levels -- the time that scientists first noticed the harmful effects human activities were having on atmospheric ozone -- sometime in the middle of this century," Yang said in a statement.

While ozone is a beneficial shield in the stratosphere, some six to 31 miles above Earth's surface, the ozone encountered at ground level can be damaging to lung tissue and plants and is a major component of smog.

The analysis was published in the Journal of Geophysical Research - Atmospheres.

Researchers from NASA and other agencies reported in June that the so-called ozone hole over Antarctica would recover by around 2068, which is some 20 years later than previously expected.

The Antarctic ozone hole is a massive loss of ozone that occurs each spring in the Southern Hemisphere.

A similar, though smaller and less severe, ozone hole has been reported in the Arctic.